1. Field of the Invention
The present invention relates to a developing device used in electrophotographic printers, and more particularly to a developing device for developing a latent image formed on a photosensitive drum with a single-component developer.
2. Background of the Prior Art
It is known that a latent image formed on a photosensitive drum is developed by a developing device. For a developer a two-component developer including a toner and a carrier or a single-component developer including a toner alone is used. In recent years a single-component developer is preferred to the two-component developer because of the non-necessity of adjusting the mixing ratio of toner and carrier, and the non-possibility of the toner being unfavorably influenced by the deteriorating carrier.
A developing device using a single-component developer is equipped with a developing roller for feeding a toner charged with a predetermined polarity. The amount of the developer to be fed is controlled by a blade made of an elastic material such as rubber, thereby forming a thin toner layer on the developing roller. When the toner layer on the developing roller reaches a face-to-face position with the photosensitive drum, the developer is transferred to the image-forming area so as to develop the latent image.
In the developing device using a single-component developer it is essential to cover the surface of the developing roller with an evenly thin toner layer, otherwise it will not be possible to develop the image on the photosensitive drum with equal density. As described above, the blade used to control the amount of toner to be fed is axially slid on the whole surface of the developing roller. However, it is difficult to enable the blade to keep contact on the developing roller with a constant pressure throughout the whole surface of the peripheral surface of the developing roller. In recent years, the developing roller tends to have a reduced diameter such as 20 mm, and it becomes difficult for the blade to keep full contact with the whole surface of such a relatively small roller. As a result, the toner layer becomes uneven, and the latent image cannot be developed with equal density.
Japanese Laid-Open Patent Publication No. 61-179473 discloses a container for containing a two-component developer, that is, a toner and a carrier of magnetic particles, and a mesh for allowing the toner alone to pass to a developing roller where an image is developed. A problem arises when a black solid image is formed with a relatively large amount of toner, the carrier having relatively large particles tend to clog the mesh, thereby causing a shortage of toner supply to the developing roller. As a result, the formed image is unclear.
In the case of a single-component developer without a carrier, no such problems arise. However, it is difficult for a sufficient amount of toner to be supplied to the developing roller because of the absence of a carrier which otherwise would be effective to entrain the toner to the mesh. As a result, an unclear image results.
Japanese Laid-Open Patent Publication No. 56-123554 discloses a developing device which allows a single-component developer to pass through a screen. In this developing device the screen keeps contact with the peripheral surface of the developing roller, and the toner passing through the screen forms a thin layer on the developing roller. In order to enable the toner to pass through the screen, a high pressure is required. The toner used in a single-component developer has relatively large particles such as 10 .mu.m in diameter. Such large particles are likely to cause a choking trouble in the screen. In this situation, if the toner is pressed to the screen at a high pressure, the toner is gathered into masses, thereby causing a secondary choking problem. Owing to the shortage of toner supply resulting from the choking, an evenly thin toner layer fails to be formed on the developing roller. If the pressure is reduced, the screen becomes safe from the choking problem, but the amount of toner that passes through the screen is decreased. A shortage of toner also occurs, and the density of the resulting image becomes inadequate. In the developing device disclosed in Japanese Laid-Open Patent Publication No. 56-123554 the toner forcing pressure is constant, but as the amount of toner changes, the required pressure accordingly changes, thereby changing the supply of toner to the developing roller. As a result, the density of the resultant image becomes uneven.